When you're looking for trout in stillwaters, there's two types of
structure to consider, submergent and emergent structures.

Emergent structure would be things like sunken branches, emergent
vegetation, lily pads, beaver lodges. These are areas that are going to
attract bait fish, aquatic nymphs, and other invertebrates. Examples of
submergent structure include weed beds, sunken islands, humps, depressions,
troughs, areas of transition, any irregularity that's going to attract and
hold trout. These are the areas we want to look for, search out, and
explore with our flies.

A lake's character lies beneath the water's surface. Although a lake's
structure is not visible at first glance as it is on moving water, it is
still there if you know what to look for. You can use the surrounding
topography as a guide to what lies beneath. A shallow shoreline would
indicate shoals and weed beds. A steep sided shoreline suggests a quick
drop into deep water. Underwater contour or bathymetric maps are available
from many locations via the Internet. These can be studies prior to arrival
to obtain a mental picture of a lake's structure.

Once on the lake, sounders are invaluable tools for locating subtle
structure nuances not visible on larger scale bathymetric maps.